An AC surface discharge type panel that is typical as a plasma display panel (hereinafter abbreviated as a “panel”) includes a number of discharge cells formed between a front plate and a back plate arranged so as to face each other. The front plate includes a plurality of pairs of display electrodes each composed of a pair of scan electrode and sustain electrode formed in parallel with one another on a front glass substrate, and includes a dielectric layer and a protective layer formed so as to cover the display electrodes. The back plate includes a plurality of data electrodes formed in parallel with one another on a back glass substrate, a dielectric layer that covers the data electrodes, a plurality of barrier ribs formed in parallel with the data electrodes, respectively, on the dielectric layer, and phosphor layers formed on a surface of the dielectric layer and side surfaces of the barrier ribs. Then, the front plate and the back plate are arranged to face each other such that the display electrodes intersect with the data electrodes in three dimensions, and then sealed. An inside discharge space is filled with a discharge gas. The discharge cells are formed at respective portions at which the display electrodes and the data electrodes face one another. In the panel having such a configuration, a gas discharge generates ultraviolet rays, which cause phosphors of R, G and B to be excited and to emit light in each of the discharge cells, so that color display is performed.
A general method of driving the panel is a sub-field method, in which one field period is divided into a plurality of sub-fields and the sub-fields causing light emission are combined to perform gray scale expression. Patent Document 1 discloses a new driving method, which is one method of the sub-field method, which improves a contrast ratio by reducing light emission that is not involved in gray scale expression to the minimum to suppress an increase in black luminance. Brief description will be made of the driving method.
Each sub-field has a setup period, a write period and a sustain period. Either one of a setup operation for all cells and a selective setup operation is performed in the setup period; the setup operation for all cells causes setup discharges in all the discharge cells that perform image display, and the selective setup operation selectively causes the setup discharges in the discharge cells in which sustain discharges have been performed in a immediately preceding sub-field.
In a setup period for all the cells, the setup discharges are simultaneously performed in all the discharge cells to erase the history of wall charges that have been stored on each discharge cell while wall charges necessary for a subsequent write operation are formed. In the subsequent write period, progressive-scan pulses are applied to the scan electrodes while write pulses corresponding to image signals to be displayed are applied to the data electrodes. This selectively induces write discharges between the scan electrodes and the data electrodes, causing the wall charges to be selectively formed. In the sustain period, sustain pulses are applied between the scan electrodes and the sustain electrodes a predetermined number of times corresponding to luminance weights, so that the discharge cells in which the wall charges have been formed by the write discharges are selectively discharged to cause light emission.
In discharge cells in which no sustain discharges have been induced, such as discharge cells that have been in black for several fields, however, shortage of priming causes a larger discharge time lag. Therefore, the setup discharges become unstable in the setup period for all the cells, causing excessive positive wall charges to be stored on the scan electrodes. The sustain discharges are induced in the discharge cells having the excessive positive wall charges stored on the scan electrodes, even though the write discharges have not been induced. These sustain discharges are visually recognized as bright spots, thus deteriorating black display quality.
Patent Document 2 describes a driving method that solves such a problem that the bright spots are visually recognized in the discharge cells having the excessive positive wall charges stored on the scan electrodes.
Brief description will be made of the driving method. An abnormal wall charges erase part in which a positive rectangular waveform voltage is applied to the scan electrodes and a negative rectangular waveform voltage is subsequently applied to the scan electrodes is provided in the setup period for all the cells or the selective setup period. Strong discharges are induced by the positive rectangular waveform voltage applied to the scan electrodes in the abnormal wall charges erase part in the discharge cells having the excessive positive wall charges stored on the scan electrodes. The wall charges are inverted by the strong discharges, and the erase discharges are induced by the negative rectangular waveform voltage subsequently applied to the scan electrodes, thus erasing the wall charges.
However, panels with a larger display screen size that meet recent demands for display devices with larger screens have a wider range of variation in the characteristics, such as the discharge start voltage and the discharge time lag, of the discharge cells arranged on the entire screen. This causes considerable variation in the magnitude of the erase discharges induced by the negative rectangular waveform voltage applied to the scan electrodes in the above-described abnormal wall charges erase part.
In this case, the wall charges are insufficiently erased in the discharge cells subjected to particularly weak erase discharges while the wall charges are inverted in the discharge cells subjected to particularly strong erase discharges.
Therefore, after application of the positive rectangular waveform voltage and then the negative rectangular waveform voltage to the scan electrodes, a positive voltage and a dropping ramp waveform voltage are applied in this order to the scan electrodes in the abnormal wall charges erase part.
Weak discharges are induced by the dropping ramp waveform voltage applied to the scan electrodes in the discharge cells of which wall charges have been insufficiently erased by the negative rectangular waveform voltage applied to the scan electrodes, so that the wall charges are adjusted to be normal. In the discharge cells of which wall charges are inverted by the negative rectangular waveform voltage applied to the scan electrodes, the discharges are induced by the positive voltage subsequently applied to the scan electrodes such that the wall charges are inverted, and the weak discharges are then induced by the dropping ramp waveform voltage applied to the scan electrodes, thus adjusting the wall charges to be normal.
As described above, the positive rectangular waveform voltage, the negative rectangular waveform voltage, the positive voltage and the dropping ramp waveform voltage are applied in this order to the scan electrodes in the abnormal wall charges erase part. This causes the wall charges to be erased by the negative rectangular waveform voltage applied to the scan electrodes in the discharge cells in which the excessive positive wall charges are stored on the scan electrodes. In the cells in which the wall charges have not been erased by the negative rectangular waveform voltage, the wall voltage is adjusted to be normal by the dropping ramp waveform voltage applied to the scan electrodes. In this manner, the state in which the excessive positive wall charges are stored on the scan electrodes is resolved, preventing generation of the bright spots.    [Patent Document 1] JP 2000-242224 A    [Patent Document 2] JP 2005-326612 A